Ok I'm going to share some ideas here that I've been keeping quiet about, since I'd rather if they didn't end up in somebody's proprietary design. However, people are starting to get in trouble over using long ground wires to get some height, so silence isn't good if maybe the idea could possibly help somebody.
First off, about height. Rich has pointed out repeatedly that height isn't so helpful for AM frequencies, especially with a short antenna. And in theory I'd agree. But few of us live on a nice, level, unobstructed test field. Stone walls, foundations, buildings (which are full of wiring and plumbing), trees.. At least getting high enough in the air to be at least somewhat clear of those would be helpful. Even if it didn't give more range, it could give a more even coverage pattern within range. Now, as Rich has pointed out, with an electrically short antenna, the rediation pattern drops off as you move away from the horizontal plane when the radiator is vertical. So "as high as possible" isn't necessarilly any practical advantage, but high enough to be reasonably clear of obvious obstacles could help.
I've been doing some tinkering. I don't have an AM part15 xmitter, but I've cobbled together a junkbox signal generator from an old portable AM radio with a little piece of stiff wire soldered to one terminal of the IF transformer. It "gets out" about 10 ft with an unmodulated signal I can pick up with my old "all band" radio if there's nothing but air between them. Laughable perhaps to people here with AM stations, but good enough for testing/tinkering and definitely unlikely to cause interference.
Ok, now if a refrigerator is between that "transmitter" and the receiver.. well it normally manages a signal of about 3 "s units" on the reciever at 10 ft. If the refrigerator is between them, no clear signal sufficient to move the meter gets through. It doesn't matter if the fridge is plugged in (and hence electrically grounded) or not. If it is between the transmitter and receiver it definitely has the effect of drastically reducing the signal. Similar results with a steel desk, which is much less metal mass then a fridge.
Some of you are no doubt saying "Well *duh* Daniel.. What did you expect?" Well, having given a lot of thought to what Rich said about height making no difference, I wasn't sure what to think, so I cobbled some junk together and made some tests for myself. As picture it worth a thousand words, likewise you can learn more from making some experiments than from just talk.
Now think of your house and your neighbors' houses. A lot of metal in there too. If you add up all the plumbing and electrical wiring *and* everything plugged into that electrical system, I'm pretty sure all that metal (grounded metal, even) would provide some obstacle to the signal. I haven't been able to test it on a house, since the range is too small. But looking at it logically, I'd bet houses make a difference. Probably other building materials in a typical house also would be factors.
So while I'd agree with Rich on the long ground wire (assuming it isn't blocked at rf frequencies with a choke, etc, but in most cases except the Talking House, it probably isn't) being a lot of the reason for greater range with height, I'd hypothesize that enough height to be reasonably clear of at least the worst physcial obstacles would likely be helpful enough to be desirable. Other reasons apply too. I haven't heard vandalism mentioned much here, but I'd think something like a transmitter, antenna at ground level would be an easy target for vandalism (or theft, with copper prices these days).
Anyway, there could be definite reasons mounting a ways up in the air would be desirable.
Ok, so back to:
“The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.”
Well, let's take that an item at a time.. Transmission line has already pretty much been done away with. Most part 15 AM rigs have the antenna mounted pretty much right on the transmitter just so one doesn't have to give up antenna length for it.
The antenna? Well, I'm pretty sure we need that.. so let's keep it.
Which brings us to "ground lead (if used)".. Well, what happens if we don't use it? The antenna "needs something to work against" because it's current/voltage flow that produce an rf field. But it doesn't *have* to be wire going into dirt, it can be a counterpoise like the "ground" side of a dipole. As has been mentioned here many times by Rich and Ermi, dipoles will work without connection to a ground.
So say we took half of that 3M for a counterpoise. I can hear you now.. "Eeek! Make an already short antenna *half* as long???" Well sure. The "ground wire" (or counterpoise, to use a better word in this case) radiates just as good as the antenna, remember? So we aren't *really* giving up half the radiating system. We're just using it to allow for the flow of current, just like always.
"It'll never work. It will be too short to be of any use. Yes, too short.." Well Yoda, so is the antenna, and you already solved that problem by an appropriate loading coil to make it resonant at the AM BCB frequencies. Sometimes referred to as an 'artificial ground' or 'tuned counterpoise', the idea of adding a tuning circuit to a counterpoise has been around for quite a long time.
"But, AM BCB is *groundwave*!" Yes it is. And if you do some math, you can also figure out that even though a part15 AM antenna is very small, the wave it puts out is comparatively *huge* in size. It's going to couple to the ground at any reasonable height, the difference between 25 ft and 2.5 inches doesn't look like that much if you're a wave over 600 ft long (at 1600 khz). Remember the airplanes using frequencies a few multiples up from AM BCB frequencies for beyond-the-horizon communications? The only way they could do that is if the groundwave still happens if you're touching dirt or not. Especially over the ocean where they got better range because "ground" conductivity was really good there.
Let me tell you about a small transmitter back in the late 1950s. It was very small, I forget the exact wattage, but less than 2 watts. It had antennas that were very electrically short for it's frequencies (20 meter and 40 meter). But it was heard by countless people about 80 miles away. It was in orbit. Sputnik 1. So electrically short transmitter/antenna systems with no connection to the dirt ground definitely do have some longstanding basis as "possible" for getting some range. Yeah, part15 power is lower and we aren't wanting just a CW "beep", but a small fraction of that 80 miles is logically *possible* even on the longer AM BCB wavelength.
Ok, there are some definite problems.. What about the power lines and audio lines acting as long groundwires? Get rid of them. Seriously, solar cells and nice rechargeable batteries have come a long way over the decades and often can be found in electronics surplus catalogs.
But the audio line? Ok, let's get wild here. STL. RF STL like microwave would be maybe a bit bulky and take battery power we might prefer not to spare. So go for a Buck Rogers solution. Laser. I'll bet some of the kids on your block have laser pointers. Not real hard to use a laser beam to carry audio, it's a fairly common school science fair project. Try this.. Take a laser pointer and wire a *small* audio frequency amplifier to it. Clip test leads across the speaker or walkman headphone element with a capacitor to block DC from the laser's batteries from going back into the amp parallel with the batteries. Then hook a small solar cell across the input of a small audio amplifier and speaker or headphone. Now put some audio into the amp on the laser. Congratulations, you've just reinvented laser light-beam communication. Quite reliable for short-range line of sight, and you can get quite good audio quality with it with a little tinkering. Now for a transmitter, that solar cell could feed the audio input (at least with a transistor or IC amplifier to get the right amount of signal). If you wanted to get real snazzy, redesign the audio amplifier in the modulation section of the transmitter to use a phototransistor in place of whatever transistor currently is doing the job. Then you could even feed it with a single fiberoptic line (which also can be inexpensive).
"Ok, if it's way up there.. and the power supply is up there.. How can I turn it on without a switch and a wire?" Well, you aren't going to be modulating that laser beam to "full dark".. so use a transistor switch so that the main power supply only switches on when the laser itself hits your sensor. It could have it's own little battery and solar cell to keep it charged if you want the main batteries to be really "tight" with juice and not spare even as much as you'd have to take to run that little sensor circuit.
Anyway, that would be one possible approach to a "no dirt ground" system. And that's a good-sized chunk of what I've been considering as a "dream station" to build. If any of the ideas can help anyone avoid a fine, you are welcome to them
Daniel
Daniel,
*Here* is a link to a table I put together a few months ago showing the performance of several possibilities, including your thoughts about a shorter whip with a radiating "counterpoise" for a total of 3 meters (no ground). That configuration is just a short dipole. Note how high its feed point impedance is, and the loss required to bring it resonance and provide good bandwidth.
//
An excellent chart, I've already seen it. However, what I'm talking about would be a dipole with loading on both elements. In fact, that chart is one of the things that got me thinking in this direction because I noted you appeared to be comparing a loaded vertical to an unloaded dipole. It seemed at the time an "apples to oranges" comparison. So I started looking into counterpoises and "artificial grounds" such as Hams use (even with qrp gear) where good physical grounds are not practical/possible, such as the hams that live in apartments.
Admittedly, by all accounts an "artificial ground" does not seem to work as well as an actual good ground, especially on lower frequencies.. But it might work better than no ground or counterpoise when mounting a part15 AM antenna higher than ground level, and it would not be outside compliance with the 15.219(b) rules and as such seems a likely place to start looking for workable answers.
Dipoles with both elements either coil loaded or matched via tuners have been in use by urban hams on the 160 Meter band for quite a long time.
Daniel
I noted you appeared to be comparing a loaded vertical to an unloaded dipole.
Daniel,
All of the antennas in that chart were coil-loaded to resonance.
Adding "capacitive top hats" to the ends of elevated dipole elements or using elevated radials with an elevated 3-m whip (with or without a "ground wire") gets into Part 15 radiator length issues, probably.
//
I hadn't considered capacitive hats, but with the transmitter mounted between the elements, it's possible that even though it would be comparatively small it *might* be viewed as adding at least some capacitive effect if it is made of metal. Probably best to make the elements under 3m (total length of the two elements combined) to allow for the width of the transmitter box.
I am not thinking of elevated radials, the idea is (as you correctly stated) basically a dipole.
I am also not looking for something with a range of 2 or more miles. 1/2 or 1/4 mile could be sufficient for a neighborhood/community station of the sort I'm envisioning as the objective for this project, if there is a way it can be done without heading into grey areas that result in people getting fines.
Daniel
But the audio line? Ok, let's get wild here. STL. RF STL like microwave would be maybe a bit bulky and take battery power we might prefer not to spare. So go for a Buck Rogers solution. Laser. I'll bet some of the kids on your block have laser pointers. Not real hard to use a laser beam to carry audio, it's a fairly common school science fair project. Try this.. Take a laser pointer and wire a *small* audio frequency amplifier to it. Clip test leads across the speaker or walkman headphone element with a capacitor to block DC from the laser's batteries from going back into the amp parallel with the batteries. Then hook a small solar cell across the input of a small audio amplifier and speaker or headphone. Now put some audio into the amp on the laser. Congratulations, you've just reinvented laser light-beam communication. Quite reliable for short-range line of sight, and you can get quite good audio quality with it with a little tinkering. Now for a transmitter, that solar cell could feed the audio input (at least with a transistor or IC amplifier to get the right amount of signal). If you wanted to get real snazzy, redesign the audio amplifier in the modulation section of the transmitter to use a phototransistor in place of whatever transistor currently is doing the job. Then you could even feed it with a single fiberoptic line (which also can be inexpensive).
Has anyone attempted this? I have been looking at engineering information and watching Youtube video and trasmission via laser and optic looks super easy. So far I'm gathering everyone just runs a wire out to their transmitter, with the exception of Carl.
Laser would offer fantastic audio quality but I have to wonder how well it would work when it rains or if someone walked in the path.
Not the only solution.
In one of my many experiments, I put a small Internet radio inside a weatherproof box with the transmitter (a Decade MS-100, FM, but it could just as easily have been AM). The box had distributed power inside, and I used a power cable to an outside receptacle.
While the power cable doesn't fit into the scenarios being described, the point is that I captured a stream with the Internet Radio, and fed the transmitter with the line level output from the internet radio (a Roku M1001). It worked quite well.
Replace the power cable with solar power, and you have your wireless, solar powered, outdoor transmitter.
I had experimented with a Roku using the shoutcast channel with good success years ago (I'm pretty sure I discussed it here a few years ago) .. I call it a "poor mans Barix". I'm not sure what the model M1001 Roku is, but mine was an older model RT which had rca jack outputs which simplified things in my case. I simply used Roku's Shoutcast channel to pick up the stream which inturn fed the transmitter.
My experimentation was rather casual and I did not really take it through the ropes, but I did let it run consistently for at least several days.. occassionally (not sure how often) it would stop but it always would pick back up by itself without any intervention (I blamed it on the fact that I was utilizing a public wifi (bar next door) for the internet connection, but I believe with a more reliable connection such cut outs would be a lot less likely.
There's also (or was) an app you can download which provides some remote access to the Roku and even lets you program it to automatically startup on boot to a particular shoutcast stream.. So if your signal should drop you could access the Roku from a computer and force a reboot, once it reboots it would then resume the specified shoutcast stream on it's own.
There probally are other channels besides shoutcast to use with the Roku for this purpose that might even be a better option, but I know for sure it will work with shoutcast.
This topic drifted around, but I'm going to post the following interesting bit in relation to the actual title of this thread.. The ground lead..
The following is excerpted is from the Highway beautification: public hearings before the Commission April 1972 , This was around the time whip and mast part 15 transmitters first went into popular use at rest stops, state parks, and a few private businesses. The highway dept then became interested in use of such transmitters and begin looking into it.. This was 6 years before TIS became into existence
Around page 550 the FCC speaks to the highway dept. (thats where the following quoted paragraphs came from), a few testimonies after that the Info Systems Inc. spoke (first manufactures of these type part15am xmtrs). You would probally enjoy reading both those proceedings, you can read/download pdf free via google books (link above)
Anyway, as for the ground lead; it was not written into law until 1974, but it's interesting to note that "the ground lead, if used" is specified during these proceedings which took place in 1972. The state highway departments wanted to know how far they can get away with in regard to broadcasting part 15AM..
George Petrutsas, of the Federal Communications Commission:
"Questions have been raised as to possibilities of operating low
power roadside transmitters in this band without diverting any
frequencies from the broadcast service. Part 15 of the Federal
Communications Commission's rules permits operation of very low
power communications devices in this and certain other bands
without a station license from the Commission, on condition that
they cause no harmful interference to the service to which the
frequencies in the particular bands are primarily allocated.
To make reasonably sure that these low power devices do not, in
fact, cause interference to broadcast operations, the Commission
has imposed rather stringent technical requirements.
To give you an idea, by way of illustration: one of the
alternatives under Part 15 requires that a communication device
operating in the standard broadcast band may not use more than
100 milliwatts of power, the antenna must be no longer than 10
feet, and this length includes not only the antenna proper but also
the length of the transmission line and the length of the ground
lead, if one is used.
As a result, such a Part 15 device in the broadcast band should
not be expected to send a reliable signal beyond about 300 to
perhaps 500 feet. Automobiles travebng at 50-70 miles per hour
would pass the coverage area of one of those transmitters before the
message is completed. Therefore, we believe that single Part 15
devices cannot be used to provide reliable roadside communications
service to motorists on the highway.
From time to time, the Commission has received requests from
manufacturers of radio devices for waiver or modification of the
technical requirements of Part 15 so as to permit transmissions of
stronger signals for roadside radio and other similar purposes.
With the exception of few experimental programs of short
duration, the Commission has consistently refused to do so,
primarily because of the interference potential and also because it
has been reluctant to encourage development of equipment and
investments in systems which have very limited potential for
successful use...
...later..
....Further studies are necessary, we believe, to explore these and
other related questions and to determine to what extent, if any,
roadside radio can substitute road signs on the highways.
Thank you.
MR. BLOOMINGDALE. I think that is the most exciting thing
we have heard in a long time. We have been through a lot of
meetings and have heard about how to pull down billboards and so
on. As I understand it, you are saying that technically it is possible
right now to broadcast at very low frequency, broadcast, say, 300
feet on the side of an intersection, and if that can be raised another
hundred feet or to 750 feet—but don't give me technical details.
Could you do it for a thousand feet on each side of the intersection
without interfering with anything?
MR. PETRUTSAS. That is the problem. Technically you can do
it at any distance. Our present rules of the Commission provide that
once you go beyond that three to five hundred feet, you get into
the areas of interference with the broadcast service and whatever it
serves. At that point you get into serious problems...
..Later..
...MR. WRIGHT. The Park Service, as I understand, has a similar
experimental program in use in Yellowstone National Park. Are you
familiar with that?
MR. PETRUTSAS. I understand they do have but I do not know
the details.
MR. WRIGHT. I guess you would have some problems, like what
kilocycles, where to put it on the band, how not to interfere with
regular commercial broadcasting.
I understand that the prime time for radio is driving time really,
which now has replaced the home radio as the most prolific market
for radio advertising, so obviously people do listen to their radios,
but do you anticipate there could be some kind of problem or
conflict arising with the regular commercial broadcasting?
MR. KASSENS. This is one of our concerns, sir. If you are
talking in terms of establishing, in effect, a government radio
station, there will be great concern about competition with private
broadcasters. On the other hand, one of these stations would have
great problems. People driving down the highway listening to their
favorite program are going to have to give it up to shift over to the
information they want. It is a problem, as Mr. Petrutsas suggested.
There has to be considerable research in this area, exactly the
type of programming that would be on it.
MR. BLOOMINGDALE. If I am driving, and it is getting along
toward dusk, I want to know what motel I am going to stay at and 1
want to know the quality of the motel. Also I want to have this
information when I am 2,000 to 3,000 feet from the intersection. I
am going to turn on my radio because I want to know where I am
going to stay, and there is no way to find out. But it is only for a
few minutes that I want to listen to this broadcast. I don't visualize
it as a news station or music station, but one that gives me practical
directional information and travel information. I don't think you
are going to ruin a radio station.
MR. WRIGHT. For purpose of the record, would you identify
yourself?
MR. KASSENS. I am Harold Kassens, Assistant Chief of the
Broadcast Bureau.
MR. WRIGHT. Are there other questions?
If not, we certainly thank you.
I wonder if we might at this time have a brief recess. Perhaps we
could recess until 4 p.m.
(Short recess)
It is interesting that even back in 1972, the expected range of a Part15.219 station (or whatever the section in the rules was back then) was 300-500 feet. Anything more than that and there were interference concerns.
Not that much different than Part15.239 FM today.
But it is possible with a good, legal (ground mounted) Part15.219 installation to achieve at least 1 mile of range to a good receiver. I wonder if someone just dropped the ball when the rules were conceived, receivers have vastly improved since then, or whether there is just some communications confusion as to what constitutes a listenable signal.
When we talk about getting 1 mile range, we're looking at a very sensitive car receiver, and perhaps just barely hearing it above the noise. Maybe that 300-500 feet being discussed was for a clear, static-free signal (i.e., city grade), and that a noisy signal would of course be heard much further.
I'm not sure we'll ever know the answers to those questions.
"..But it is possible with a good, legal (ground mounted) Part15.219 installation to achieve at least 1 mile of range to a good receiver. I wonder if someone just dropped the ball when the rules were conceived, receivers have vastly improved since then, or whether there is just some communications confusion as to what constitutes a listenable signal..."
That's the weird thing, they didn't drop the ball.. They realized almost 50 years ago the extended capability, and proposed correcting it, but instead the FCC knowingly upheld and maintained 15.219 despite the fact it obviously defeated the original intended imposed limitations..
Not to long ago I inquired here on what exactly is meant by the term "usable signal".. What influence me to ask was in reference to the same thing you just pointed out. There is a reason why the FCC did not eliminate 15.219 (for manufactured transmitters).
What that reason is remains elusive. But there's something there.. hiding.
Case in point; in the 1976 version of Docket 20780 shows that the FCC did not simply "drop the ball" on this point.. They were very aware of the problem, they considered it and proposed changes, as clearly illustrated within the document -- But ultimately did not to make any changes to the alternative rule at all, they left it as it was ..
https://digital.library.unt.edu/ark:/67531/metadc306575/m1/688/
Excerpts:
Notice of Proposed Rulemaking to amend Part 15 of the Rules to
redefine and clarify rules governing restricted radiation devices
and low power comm. devices. Changes would make devices with
a high potential for interference subject to equipment authoriza-
tion program. The current technical limit on such devices is no
longer adequate.
F.C.C. 76-347
In the Matter of
AMENDMENT OF PART 15 TO REDEFINE Docket No. 20780
AND CLARIFY THE RULES GOVERNING
RESTRICTED RADIATION DEVICES AND LOW
POWER COMMUNICATION DEVICES.
NOTICE OF PROPOSED RULEMAKING
(Adopted: April 14, 1976; Released: April 23, 1976)
...Part 15 is based upon the rationale that a device complying with
the provisions contained therein will, in general, not be a source of
harmful interference to radio communications. The technical limita-
tions in Part 15 are designed to ensure this by setting a fixed maxi-
mum radiation for various devices. An additional requirement that
they do not cause interference is placed on the operation of these
devices in recognition of the fact that even at the extremely low radi-
ation limits they will in some circumstances cause interference. This
means that the operator of a Part 15 device is obliged to correct any
interference that is caused, even when the device is in compliance with
the technical specifications in Part 15. ...
...In lieu of meeting this radiation limit, a low power
communication device operating on a frequency in the bands 160-190
kHz or 510-1600 kHz may as an alternative meet the antenna and
power limitations in Sections 15.112 and 15.113, respectively. These
provisions were intended to make it feasible for a radio enthusiast to
construct and operate a low power radio, since it is reasonable to
assume that the average hobbyist or experimenter would not have the
capability of making field strength measurements. It has come to our
attention, that manufacturers are taking advantage of the alternative
provisions to market devices, which, because of the greater efficiency
of the new technology, actually operate with radiated signals in excess
of those permitted by Section 15.111. Since the interference potential
of a device is directly related to the field strength of the radiated field
and not the power fed into the antenna, and since a device which is
widely marketed to the general public has a much greater potential for
causing interference than a device built by an individual for his own
use, we are proposing to restrict the alternative provisions to home
built devices. ....
....The marketing rules and the equipment authorization pro-
gram are designed to protect the unsuspecting consumer by insuring
that a RF device which is widely distributed will not become a source
of harmful interference. This is based on the consideration that it is
easier to prevent interference by checking the interference potential
of the device at the source (manufacturer), rather than to correct the
problem once the device is widely distributed to the public. Moreover,
prior to the adoption of these rules, the Commission was often forced
to move against the operator (typically a consumer) when it was dis-
covered that his device, which was purchased in good faith, was caus-
ing harmful interference.....
--------------
Edit-- I don't know why words get run together when I copy and paste content here (like it has above), I think it's still readable anyway, but if you have problems reading it then just click the link instead
ARTISAN SAID: "Maybe that 300-500 feet being discussed was for a clear, static-free signal (i.e., city grade), and that a noisy signal would of course be heard much further."
I said: " I know it' subjective but I make that assumption as well."